1. Field of the Invention
The present invention relates to a stereoscopic image display apparatus using a parallax barrier method and, more particularly, to a bright stereoscopic image display apparatus.
2. Related Background Art
A stereoscopic image display method using a parallax barrier method is disclosed in S. H. Kaplan, "Theory of Parallax Barriers", J. SMPTE, Vol. 59, No. 7, pp. 11-21 (1952). With this method, a stripe image formed by alternately arranging right and left image portions of a plurality of parallax images is observed by an observer via a slit array (called a parallax barrier) which is arranged at a position separated by a predetermined distance from the stripe image and has predetermined aperture portions and light-shielding portions, so that the two eyes of the observer can observe the corresponding parallax images, thus attaining stereoscopic viewing.
This stereoscopic image display method is well known as a method that requires no spectacles. However, since this method has the following drawbacks, it is very rare to realize this method as a practical stereoscopic image display apparatus.
In the conventional parallax barrier method, a linear grating-shaped element on which pairs of aperture slits and light-shielding barriers are arranged is used. In order to reduce crosstalk between the right and left parallax images, the light-shielding barrier and the aperture slit preferably have high contrast therebetween. More specifically, the light-shielding barrier preferably has a transmittance closer to 0%, and a light-shielding barrier having a transmittance of 1% or less can be realized by depositing a metal film (e.g., aluminum, chromium, or the like) on a glass substrate. When the duty ratio between the widths of the light-shielding barrier and the aperture slit, in other words, the width of the aperture slit with respect to the grating pitch is defined to be a slit aperture ratio, the slit aperture ratio is normally set to be 50% or less since a large crosstalk reduction effect can be expected as the light-shielding barrier has a larger ratio. By definition of luminance, the slit aperture ratio has no influence on luminance, but the width of the slit is approximately equal to the pixel pitch. For this reason, rather than this microscopic luminance, the average luminance on the entire screen, which corresponds to the brightness perceived by the observer and is normally used in a display apparatus will be examined hereinafter. More specifically, since the light-shielding barrier has substantially zero transmittance, and the slit aperture ratio is 50% or less, the original brightness of the stripe image is lowered to 1/2 or less in principle in the conventional parallax barrier method.
Furthermore, a viewing region which is determined by the geometric layout and allows stereoscopic viewing is narrow. Since a stereoscopic viewing region and an inverse stereoscopic viewing region are alternatively aligned in the horizontal direction, the observer must move his or her head so as to adjust his or her pupil positions or view points to fall within the stereoscopic viewing range. This problem depends on the widths of the light-shielding barrier and the aperture slit, and a region suffering large crosstalk between parallax images is present between the stereoscopic viewing region and the inverse stereoscopic viewing region.
As described above, in the conventional parallax barrier method, the brightness of an image is lowered to 1/2 or less due to the presence of the light-shielding barrier, and a dark image is destined to be displayed. This problem worsens when a liquid crystal display apparatus (to be referred to as an LCD hereinafter) is used as a display apparatus. That is, in this case, due to the low aperture ratio (30% to 60%) of pixels of the LCD, a darker image is displayed. For this reason, the luminance and consumption power of a flat backlight light source as an illumination device of the LCD must be increased, and in a notebook type personal computer or a palmtop type personal computer with high portability, it is almost impossible to adopt the parallax barrier method in terms of the service life of a battery.
In order to solve the problem of the narrow viewing region, a stereoscopic display method using a linear array of columnar lenses (called a lenticular method) is widely known as a method of broadening the viewing region. However, since a stripe image is observed via columnar lenses serving as micro phase objects, images in micro regions are distorted due to the lens effect (e.g., astigmatism) of the columnar lenses, resulting in glittering and unnatural feeling. Thus, this method also suffers a problem in terms of its principle, i.e., it is hard to obtain a high-quality image.